Water Parameters within the Reef Aquarium – A Hobbyists Guide to pH

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geologeek
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Water Parameters within the Reef Aquarium – A Hobbyists Guide to pH

Postby geologeek » Thu Oct 08, 2009 12:36 pm

Now pH is one of those things that people often get confused about and as a result spend an inordinate amount of time worrying about it.

pH is just a representation of the quantity of hydrogen ions in a body of water. The logarithmic scale used to measure pH means that we see changes in [H+] that are orders of magnitude different. For example a pH of 1 is ten times as acidic as pH 2 and one million times as acidic as pH 7 (you with me still?)

Well to save me time I have taken this from our friends at PFK: Here

The basics

Although it's not completely unfathomable, the pH scale is actually considerably more complex than it appears. After all, pH doesn't really exist - it's kind of abstract in a way, and is merely supposed to be a quick and easy way of expressing the minuscule concentration of hydrogen ions dissolved in the water.

The more hydrogen present, the more acidic the water - the more hydroxyl, the more basic (or alkaline) the water is.

For example, it is much simpler to see that a pH of 6.5 is more acidic than a pH of 7.8 than it is to compare the tiny molar equivalents of hydrogen 3.1622776601684E-07 and 1.5848931924611E-08 per litre of water...

The pH scale was developed by Danish biochemist Soren Sorensen in the 1800s and quickly became the accepted method of expressing the acidity of solutions. Roughly speaking, pH stands for pondus hydrogenii which translates to "potential hydrogen".

Pedants may be interested to know that although the correct way of writing pH is with a lower-case "p", Sorensen himself used capital letters (PH) in his original paper. It wasn't until WM Clark's papers in 1920 that people started to adopt the current form, pH.

The scale extends from 0-14 with 0 being extremely acidic and 14 being extremely alkaline. The midpoint, pH 7.0, is neutral so the water is neither acidic nor alkaline. At pH 7.0, there are equal amounts of hydroxyl and hydrogen ions in the water.

pH pitfalls

The weird thing about the pH scale is that it's logarithmic and the scale is essentially upside down. Basically, all this means is that there's quite a big chemical difference between pH values that look like they're fairly similar to each other. At the acidic end of the scale there is loads of hydrogen, but at the alkaline end there is very little.

Each major point on the scale, such as 4.0, 5.0 or 9.0, represents a tenfold difference. A pH of 6.0 is ten times more acidic than pH 7.0, and pH 8.0 is ten times more alkaline than pH 7.0. Bigger jumps are even more dramatic.

A pH of 6.0 is 100 times more acidic than pH 8.0, and a pH of 5.0 is 1000 times more acidic than pH 8.0.

In technical terms, the pH of a solution is equal to the negative logarithm of its H+ concentration, so [H+] = 1 x 10-pH and pH = log [H+].

What seem like little changes on the scale are masking much bigger chemical changes. Seemingly small shifts in pH can have quite a dramatic effect on the fish, so try to keep the pH stable. Unfortunately, quite a few things can make it go up or down like a yo-yo, and these could stress or even kill some fish. Checking the pH regularly, especially if you're dosing with carbon dioxide or using a calcium reactor, is a sensible idea.

Why?

Well, marine aquatic organisms thrive in a particular pH range which largely varies from organism to organism. It is therefore difficult to justify a claim that a particular pH range is "optimal" in an aquarium housing many species.

Even natural seawater's pH (8.0 to 8.3) may not even be optimal for some of the creatures residing within the oceans, however it was observed over eighty years ago that pH levels that significantly vary from that of natural seawater can ultimately stressful to fish. (see here)

The toxicity of certain metals such as copper on certain aquarium organisms such as mysids and amphipods, is known to vary with pH (see here). As a result the acceptable pH range of one person’s aquarium may differ from another, even if they contain the same organisms, if they fundamentally have different concentrations of metals.

Yeah I thought that might confuse you! and trust me the same happens in the freshwater habitats as well as in the ground and has taken me many years to figure out how these processes work! Let’s leave it at the fact that pH can have an affect on the toxicity of metals……..and these can be countered by the presence of say calcium and alkalinity – just to confuse things further! (read here)

Changes in pH therefore substantially impact some fundamental processes taking place inside many marine organisms. One of these processes is calcification (deposition of calcium carbonate skeletons) which is known to be pH dependant.

As there are so many variables involved, one cannot say a pH of 8.1 is better than a pH of 8.2! I would however suggest that the pH of natural seawater (given Mother Natures role in the grand scheme of things) of approximately 8.2, is the most appropriate.

However, as we all know our aquaria can succeed under a wider range of pH values in the range from 7.8 to 8.5 - It is large or sudden swings that we need to elliminate.

It should be CLEARLY stated however, that alkalinity must be at least 2.5 meq/L or higher at the lower end of this pH range (any problems associated with calcification at lower pH values may just be offset by having a higher alkalinity). In addition calcium would need to be maintained ideally at a minimum of 400 ppm.

Conversely, one of the problems at higher pH (generally above 8.2) is the abiotic precipitation of calcium carbonate which can have a drastic effect in dropping calcium and alkalinity levels within the aquarium or even blocking the impellor of a pump.

In the open ocean pH has been observed not to vary much over the course of a day, and averages around 0.02 units. Around coral reefs there is a far larger density of organisms than can be seen in the open ocean. The activities of these organisms cause the pH in the water overlying reefs to vary much more than in nearby oceanic systems.

Even on outer reefs where there is a massive exchange of water, it's not unusual to see a range of 0.2 pH units per day (e.g., pH ~8.1-8.3 over a 24 hr period). On reef flats, in lagoons, and areas with restricted water exchange it is possible to see much more variation during the course of a day.

On some reef flats pH values have been measured to vary from as low as 7.8 to as high as 8.4 in a single 24 hr period (Yates and Halley, 2006). In some lagoons, pH has been measured to vary as much as 1 pH unit in a day (e.g., 7.6 to 8.6). Seasonal or other cycles of pH variation in reef water have also been measured (Pelejero et al., 2005).

Accordingly some of the pH values that organisms experience may be less than ideal for growth, many are able to tolerate a fairly wide range of pH values, at least for short periods of time – take rock pool inhabitants as an analogy………

pH is therefore an important parameter for us to consider for many reasons! Firstly, many physiological processes in living organisms are strongly affected by pH. If the pH of our tank water is substantially high or low, it may have direct, negative impacts on many of the organisms in our tanks, not just the calcifying organisms. In addition, pH plays a very important role in affecting the rate of CaCO3 precipitation and dissolution – which I hope to address soon :)

Go slow and let it grow!
Last edited by geologeek on Thu Oct 08, 2009 12:40 pm, edited 1 time in total.
Reason: Sorry but i cant give you access to the full papers - but many are free
Go Slow And Let It Grow! - But remember Detritus Happens!

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